This invention relates generally to gate valves and deals more particularly with an improved controlled float seat construction for expanding gate valves.
In a typical expanding gate valve, the gate mechanism expands tightly against the valve seats in both the fully open and fully closed positions of the valve. To facilitate movement between the open and closed positions, the gate mechanism collapses or contracts for the purpose of preventing the seats from dragging against it. However, since the seats are often mounted rather loosely in order to accommodate manufacturing tolerances and other imperfections in the sealing surfaces, the seats are able to "float" inwardly and bind against the collapsed gate assembly. Such binding action of the seats significantly increases the operating torque of the valve and the wear on the components, particularly the sealing surfaces. In order to avoid this problem, various types of limited or controlled float seats have been proposed, including the arrangement shown in U.S. Pat. No. 3,823,911 to Natho et al.
Although seat spacers such as those disclosed in the Natho et al patent are satisfactory for the most part, they are not wholly without problems. Most notably, a large body cavity is required to accommodate the spacers due to their size and shape and their manner of cooperation with the valve seats. Therefore, the valve body must be larger than is desirable and the cost of the valve increases accordingly. The spacers that have been proposed in the past are also ill suited for use in cylindrical body cavities which in many situations have significant advantages over other shapes.